1. Technical Field of the Invention
The present invention relates to an optical disk recording apparatus which irradiates a guide groove preformed in an optical disk with an optical beam mainly responsible to carry out information recording along the guide groove, and particularly to a compact disk recording apparatus.
2. Prior Art
When recording/playback is carried out in an optical disk recording apparatus, particularly in a CD-R recording apparatus, an optical beam is controlled to trace a track formed on the optical disk. Various control methods are available, and there is a method called a 3-beam method which uses a zero-order light as a main beam and positive and negative first-order lights as sub beams obtained by passing a laser beam emitted from a light source through a diffraction grating.
FIG. 8(A) is a view schematically showing a situation where playback is carried out from a CD-ROM which is a read-only optical disk by employing the 3-beam method. In FIG. 8(A), a track 25 indicated by a broken line is a plane, and a row of pits 21 represented by hatching is actually called a track. The pit 21 is formed in a relief structure, and projected in an upper direction from a paper surface. Among three beams, a main beam 15 which is a zero-order light is mainly responsible for information reading, and follows a track 25. The light reflected back from the track receives a light reduction effect of scattering or the like by the pits 21 to thereby contribute to the information reading. Sub beams 16 and 17 which are positive and negative first-order lights are positioned before and after the main beam 15, and arranged to be offset left and right at a 1/2 track pitch. A tracking error signal is obtained from a differential signal between both outputs of the sub beams 16 and 17, and a main beam position is controlled in accordance with the tracking error signal to carry out tracking control. An arrow in the drawing indicates a traveling direction of the optical beam relative to the track 25.
FIG. 8(B) is a view schematically showing a situation where recording is carried out in a CD-R which is a recordable optical disk by employing the 3-beam method. A guide groove 20 indicated by a solid line in FIG. 8(B) is equivalent to the track 25 of FIG. 8(A). The guide groove 20 is formed in a relief structure, and projected in an upper direction from a paper surface. During recording, among three beams, a main beam 15 which is a zero-order light is mainly responsible for information recording and follows on a track 20. In a pit portion, the laser power is increased to a recording power level to form a pit 21 and, in a blank portion 22, the laser power is controlled to a reading power level to follow the guide groove. The pit 21 formed here has a reflected light reduction effect equal to that of the relief pit 21 of FIG. 8(A). Sub beams 16 and 17 which are positive and negative first-order lights are positioned before and after the main beam 15, and arranged to be offset left and right at a 1/2 track pitch. A tracking error signal is obtained from a differential signal between both outputs of the sub beams 16 and 17, and a main beam position is controlled in accordance with the tracking error signal to carry out tracking control. An arrow in the drawing indicates a traveling direction of the optical beam relative to the track 20.
FIG. 2 schematically shows a situation of the CD-R recording apparatus during recording and tracking. With respect to a time axis, a recording signal is changed alternately to a low (L) level equivalent to a reading power level and a high (H) level equivalent to a recording power level, and the guide groove is irradiated with a laser beam modulated in accordance with the recording signal. The laser beam with which the guide groove is irradiated is reflected to reach an optical pickup again, and then converted into an electric signal by a photodetector to be outputted. An outputted signal of the reflected light becomes a fully reflected level until a pit is formed, thus exhibiting a sudden level increase. The signal is increased to Pw equivalent to the recording power level and, when pit formation is started, energy is absorbed to reduce the signal level. During the pit formation, a stable level is exhibited. When irradiation laser power becomes a reading power level in accordance with the recording signal, a reflection signal output also becomes Pr equivalent to the reading power level.
During the process from the recording signal to the reflection signal, waveform correction of the recording signal called a write strategy is carried out to set desired pit and blank lengths. The recording signal and the reflection signal are consequently deviated from each other in certain parts, but a detail is omitted.
After predetermined time from the point of time when the reflection signal becomes the reading power level Pr, a servo sampling pulse is issued, and servo sampling is carried out in a predetermined section of the reading power level Pr to generate a tracking error signal. During the pit recording by the main beam 15, among a pair of sub beams, a preceding sub beam 16 is placed on an unrecorded track, while a succeeding sub beam 17 partially overlaps the pit in which recording is currently carried out by the main beam 15. During this period, however, tracking error signal generation is not carried out. For the tracking error signal, a tracking error signal obtained in a blank section immediately before is held to control tracking. After the predetermined time from the point of time when the reflection signal becomes the reading power level Pr, the succeeding sub beam 17 is shift in a blank portion and, in this blank portion, servo sampling is carried out to generate a tracking error signal. That is, during the tracking error signal generation, since the preceding and succeeding sub beams 16 and 17 are both placed in the blank portion, no difference is generated in the amounts of reflected lights between both of the sub beams 16 and 17 while the main 15 beam follows around the guide groove 20.
Regarding the pit formation, however, if pits are formed deeply, densely or widely, recording is carried out by increasing laser power. Specifically, if recording is carried out under conditions such as 70% or higher of a recording signal duty, 20 mW or higher of recording laser power, and CAV control of a recording speed from an inner peripheral speed of 4 times to an outer peripheral speed of 10 times, most parts of the optical disk recording surface become pits, and overpower recording is carried out in all recorded areas. FIGS. 3(A) and 3(B) are schematic views when recording is carried out while increasing the laser power to widely and intensively form pits. FIG. 3(A) shows a state during the servo sampling, i.e., a state immediately before a succeeding sub beam 17 reaches a blank position to issue a sampling pulse. If a sampling pulse is issued, and a tracking error signal is sampled, the following problem is observed to occur. Compared with FIG. 2, the preceding sub beam 16 is in an unrecorded guide groove (i.e., blank portion), while the succeeding sub beam 17 partially overlaps the wide pit 21 in which high laser power recording has been carried out one round before to thereby reduce the amount of a reflected light of the sub beam 17. Then, an error occurs because the tracking error signal is a differential signal between both of the sub beams 16 and 17 and, by its impulse response, an undesirable track jump occurs.
FIG. 3(B) is a view schematically showing a relation between the observed tracking error signal and guide groove tracking. During the guide groove tracking, the tracking error signal is controlled nearly to a zero value to carry out stable tracking. However, the moment another round is made, the aforementioned tracking error signal is generated when the position of the pit widely formed at high laser power one round previously is reached and, by an impulse response to a noise signal, an undesirable track jump is caused. Depending on a magnitude of the response, only slight oscillation occurs. However, this oscillation occurs during subsequent rounds and, by this undesirable tracking error signal, a tracking servo becomes unstable.